io8 



HISTOLOGY. 



larger, often provided with a small cluster of terminal nodules, they are 

 like those of smooth muscle. They belong with the sympathetic system. 

 The accessory fibers of the vagus which enter the cardiac plexuses, are not 

 known to terminate upon the muscle fibers. 



Nerve fiber 

 bundle. 



FIG. 131. MOTOR NERVE ENDINGS OF INTERCOSTAL MUSCLE FIBERS OF A RABBIT. X 150. 



Striated muscles are innervated by the neuraxons of the ventral roots, 

 which grow out from cell bodies remaining within the central system. 

 These neuraxons, as medullated fibers, extend through the spinal and 

 certain cerebral nerves to the muscles. They form plexuses of medullated 

 fibers in the perimysium, from which branching medullated fibers pass on to 

 the muscle (Fig. 131). Each muscle fiber receives one of these branches, or 



sometimes two placed near together. 

 They are usually implanted near the 

 middle of the muscle fiber. The 

 connective tissue sheath of the nerve 

 blends with the perimysium; the 

 neurolemma is said to be continu- 

 ous with the sarcolemma, the nerve 

 having become attached to the em- 

 bryonic muscle fiber before the sarco- 

 lemma had developed. Under this 

 membrane the myelin sheath ends abruptly, and the fiber ramifies in a 

 granular mass considered to be modified sarcoplasm. It may contain 

 muscle nuclei. This granular mass with the nerve ending appears as a 

 distinct elevated area, estimated to average from 40 to 60 in diameter, 

 and has been named the motor plate. A surface view and a section of a 

 motor plate are shown in Fig. 132. 



FIG. 132. MOTOR PLATES. 



A, Surface view, from a guinea pig; B, vertical 

 section, from a hedgehog. (After Bohm and 

 von Davidoff.) g., Granular substance of 

 the motor plate; m., striated muscle; n., 

 nerve fiber; t. r., terminal ramifications of the 

 nerve fiber. 



